This invention relates to position detecting device and method for detecting the position of a mark formed on an object and, more particularly, to position detecting device and method for accurately detecting the position of a mark on the basis of image signals obtained by image pickup through image pickup means such as a TV camera or a charge coupled device, upon alignment between a photomask or reticle and a semiconductor wafer prior to exposure processes for the manufacture of semiconductor devices.
In exposure apparatuses for use in the manufacture of semiconductor devices such as integrated circuits, detecting the position of a mark prepared for the purpose of alignment between two objects involves, as is known in the art, viewing the mark through image pickup means and binarizing or binary-coding the output signals produced by the image pickup means.
The output signals of the image pickup means vary in dependence upon the kind or condition of the wafer on which the mark is formed and/or the magnitude of a concavity or convexity defining the mark. Since the degree of variation in the output signal is very high, the binary-coding often fails to meet such variation, so that a pattern other than the alignment mark is detected as if it is the alignment mark in some cases, or the detection itself ends in failure, even if the alignment mark is present in the field of detection.